Pulsed Infrared Laser Thermal Lens Spectrometry of Flowing Gas Samples

The behavior of the pulsed Infrared laser thermal lens signal Is studied for flowing gas samples. Possible sources of decreased TLS signal are discussed including flow rate and cell temperature. The TLS signal is found to be Independent of the flow rate at all flow rates studied. It Is calculated that absorbances as low as 2.5 x 10-7 In argon and 1 x 10-7 in liquid carbon disulfide should be detectable. The possibilities of using this technique as a chromatography detector for ultratrace amounts of analyte are also discussed

A Least Squares Digital Filter for Repetitive Data Acquisition

A least-squares digital filter is proposed for impulse-response data acquisition schemes. The filter operates in real time by fitting each signal transient to a filter function. The optimal filter function is found to be the signal waveform for the case of white noise. The signal-to-noise ratio improvement of the filter is proportional to the square root of the number of data points used in the filter-function waveform

Species Selective Detection in Gas Chromatography Through Photothermal Deflection Spectroscopy

The use of photothermal deflection spectroscopy is demonstrated as a species-selective detection technique for gas chromatographic analyses. The technique is illustrated by selectively detecting Freons dissolved in dichloromethane. It was also used selectively to detect two of the three isomers of dichlorodifluoroethylene. The mass detection limits for the Freon mixture range from 0.7 to 4 ng, showing the good sensitivity of the technique. Several applications in atmospheric analysis and liquid chromatography are briefly discussed

Pulsed Laser Thermal Lens Spectrophotometry of Liquid Samples Using an Optical Fiber Beam Guidewith Interference Orthogonal Signal Processing

An apparatus for thermal lens spectrophotometry of liquid samples is conshucted using a pulsed nitrogen laser excitation source and a heliium-neon laser probe. Both laser beams are passed through a fused silica optlcal fiber prlor to being tightly focused Into the sample cell. The optical flber is used to reduce mode and pointng varlatlons in the laser outputs. A dlgltal correlation filter is developed and used for estlmatlon of the thermal lens signal. This dlgltal estimation procedure exhlblts a considerably enhanced slgnal-to-nolse ratio over that of other estimation methods. Moreover, It is relatively Immune to background Interference and rapid In estlmatlon time. Finally, the effectlve sample path length for focused beams In pulsed laser exclted thermal lens spectrophotometry is examlned from a theoretical basis. It is found that the slgnal will be very near maximum for cell path lengths on the order of 4 times the Raylelgh range